Silicone rubbers are used in a wide variety of fields to take advantage of their heat resistance, cold resistance, and electric characteristics. However, since the siloxane polymers used in the production of the silicone rubber is generated by equilibrium reaction of the siloxane oligomer by acid or alkali, it is known that a considerable amount of the so called “non-functional” cyclic siloxane having no reactive group such as SiH functional group or alkenyl functional group in the molecule, namely, the so called “low-molecular weight siloxane” which has a degree of polymerization (namely, number of silicon atoms per molecule) of up to 20 would remain in the reaction product irrespective of the degree of polymerization. While such low-molecular weight siloxane can be removed by volatilizing at reduced pressure or at high temperature, complete removal has been difficult, and the resulting product always contained such low-molecular weight siloxane at a concentration of approximately 1% by weight, or at a lower concentration of about 0.5% by weight (about 5,000 ppm). Such low-molecular weight siloxane volatilizes not only in high temperature atmosphere, but it has been known that a slight amount of such siloxane also volatilizes from the cured rubber at room temperature and deposits on the surrounding surfaces causing various problems such as clouding or haze, contact fault, adhesion failure, and hydrophobicization of the surface. While such problems can be obviated to certain extent by post-curing the cured rubber at a high temperature, there are still problems such as incapability of exposing to such high temperature when the rubber is used in sealed condition or when the rubber is used in a module in combination with other less heat resistant resin.
For example, JP-A 3-157474 (Patent Document 1) corresponding to U.S. Pat. No. 5,145,931 discloses an addition curing silicone rubber adhesive wherein content of the low-molecular weight siloxane having a vapor pressure at 200° C. of at least 10 mmHg is up to 500 ppm. JP-B 6-54405 (Patent Document 2) discloses a fixing roller wherein content of the low-molecular weight siloxane having the degree of polymerization of up to 20 is up to 0.75% by weight. JP-A 4-311764 (Patent Document 3) discloses a method for producing a siloxane polymer having a reduced content of the low-molecular weight siloxane by extending the chain length using a polymer having SiH functional group at opposite ends thereof.
However, even if content of such low-molecular weight siloxane were reduced, the product still suffered from the problems as described above as well as the problem that removal of the low-molecular weight siloxane that had deposited on the surrounding surface by volatilization from the cured product had been difficult even by means of wiping with a solvent.